Bitty isn't yet available as a built-in widget on WordPress.com, but you can still copy/paste the HTML into your site (be sure to use the "IFRAME-based" code).

You can also help us get Bitty added to the built-in widgets on WordPress.com -- simply go to Automattic's contact page and suggest that they add Bitty to WordPress.com (they asked us to encourage your feedback).

For all other WordPress sites...

If you have the WordPress Widgets plugin, just follow the steps below (if not, see here).